Electrical resistance and method of making same



A g- 15, 1939- N. c. SCHELLENGER 2,169,594

ELECTRICAL RESISTANCE AND METHOD OF MAKING SAME Filed Oct. 12, 1935 2 Sheets-Sheet l INVEN TOR CHELLENGER BY flTT).

NEWTON C S Aug. 15, 1939. N. c. SCHELLENGER ,169.594

ELECTRICAL BESILTANCE AND METHOD OF MAKING SAME Filed Oct. 12, 1935 2 Sheets-Sheet 2 STEPI STE/ E .STEPE W ROLL/N6 jIC'E. 575p I STE/112" STEP .ZZ

fi f STHMPING. CUBE ROLL/N6.

JNVEN'T'OR NEWTON C SCHELLENGER.

i atented Aug. 15, 1939 UNITED STATES FATE UFFICE ELECTRICAL RESISTANCE AND METHQD OF MAKING SAIlIE Newton 0. Schellenger, Elkhart, Ind., assignor to Chicago Telephone Supply 00., Elkhart, Did, a

corporation of Indiana Application October 12, 1935, Serial No. 44,693

copending application Serial No. 732,604, filed- June 27, 1934, for Variable resistance devices, which has matured into Patent Number 2,118,112, and relates to electrical resistances and a method of making same, whereby constantly smooth, positive, uniform, and noiseless resistance variation and contact may be had in volume controls, rheostats and like devices useful in the radio and electrical arts generally.

My invention may be related more particularly to the class of resistance or control utilizing a resistive element comprised of a conducting agent carried in some form of binder or vehicle, such for example, as carbonaceous particles or graphite suspended in varnish.

Such a resistive composition results, upon the setting, curing, or drying of the binder, in an element having a characteristically, though minutely rough surface, often impossible of detection to the unaided eye, but visible under magnification, or otherwise evidenced by its unstable, erratic behavior in an electrical circuit where a wiping or similar variable contact is established with such surface.

However, the invention is not necessarily restricted to the carbonaceous type of resistor, but is equally applicable to other types, whether compositely or chemically formed, and broadly to any irregular resistive surface which can be impressed and/or rolled or distorted, substantially in the manner to be described.

In variable resistance devices employing a contact shoe or button adapted to move across a carbonaceous resistive surface, I find that the contact members do not corfiistently establish contact with the entire area engaged in any given position, but touch such surfaces indifferently at, and on, superficial points.

This appears to be due largely to the fact that the original surface of the resistive element is not itself perfectly smooth, even when treated by ordinary methodabut is minutely pitted, ridged or cratered.

Therefore, when the relatively smooth, plane surface of a contact shoe engages the relatively pitted surface of a resistance element of this class, a number of undesirable things result: among others, the tendency of the contact member or shoe to shift or rock from one minute point to another, giving rise to electrical disturbances, fluctuations, and noise in the output of the circuit under control.

There is a further tendency of the contactor to wear away the minute peaks and upstanding particles of the resistor surface and deposit them elsewhere along the contact path. Where such contact path happens to be one of changing resistance gradient, as is commonly the case-that is, where one section of the path may have a low unit resistance, while the section immediately adjacent has a much higher unit resistance-then the unit resistance value along one or another portion or region of such path will be altered and render the device useless for its designed purpose.

In cases where a high-resistance path of the order of 100,000 ohms or more must be crowded into a narrow section of the resistance element, say as much as a half inch or less of arc, the slightest shifting or fluttering of the contact shoe when in motion or at rest, due to dirt or minute resistive-surface irregularities, can cause a great change in the derived circuit resistance and derange the otherwise uniform operation of the control necessary in the sharply critical type of device now demanded in the radio and communication arts.

It is broadly an object of my invention to provide a resistance element having uniformly smooth contact surfaces and substantially unalterable overall resistivity and continuity.

More particularly, it is an object of my invention to provide by the method disclosed, a novel resistance element having surface conformations adapting it to constant, uniform, electrical and mechanical engagement by a movable contacting member. 1

Another object is the provision of a resistance element having a contactable surface of rolled configurations resulting in a plurality of contact surfaces lying in substantially concentric, radial paths relative to a contacting member.

Still another object of the invention is the provision of a novel resistance element or structure accomplishing the afore-mentioned objects, and additionally therewith the further object of preventing the migration or carrying-over of particles of the resistive substance onto other portions of the element by the contact member. It is further an object of the invention to contribute to the art a novel resistance element having a ribbed, grooved, corrugated, or lined contact surface, deformed or pressed for the purposes, in the manner, and by the method herein set forth.

It is yet a further intention of my disclosure to accomplish successfully the foregoing severalobjects in a manner particularly distinguishing from any prior, known art in which irregular,

roughened contact surfaces may have been obs5 tained or utilized, and/or in which various grooving and channeling expediencies may have been resorted to for the same or related purposes.

These and other objects residing in the invention will appear in view of the following particular description and the annexed drawings, in which like reference characters refer to similar parts, and in which:

Fig. 1 is a top plan view of one form of surfaced or processed element;

Fig. 2 is a fragmentary sectional view along the line 2-2 of Fig. 1, showing the contact ridging before it is rolled or deformed;

Fig. 2--A is a fragmentary sectional view taken along the line 2-2 of Fig. 1, similar to Fig. 2, except that the ridging has been deformed or reduced by rolling, and the view includes a contacting member;

Fig. 3 is a top plan view of an element showing a variant form of surface ribbing or grooving;

Fig. 4 is amagnified vertical section, in perspective, including a curved-faced shoe and showing radial contact along circumferential paths;

Fig. 5 is a top plan view of a mounted element having a variable resistance'gradient;

Fig. 6 is a plan view of a complete control unit including a surfaced resistance element;

Fig. '7 is a magnified section of pressed ridging, and

Fig. 7-A shows sectionally and in greater detafl the pressed marking of Fig. 7 relative to a contact shoe, similarly to the view of Fig. 2-A; Fig. 8 is a magnified fragment in perspective of a modified form of element before rolling, while Fig. 8-A shows the same fragment as Fig. 8, after rolling, in elevation;

Fig. 9 is a diagrammatic illustration of the steps of the process, and

Fig. 10 is a diagrammatic illustration of the steps in an alternative method of the process. My one method of the invention, diagrammatically shown in Fig.9, I employ a base of resinous or other insulating material I! such as Bakelite,

for example, and by a spraying or coating means I (step I-A) I apply a layer of carbonaceous or the like resistive material 16 thereto, which is allowed to set or may be cured to a state of nearplasticity by any usual means, as in the gas oven I3. (My application filed November 2, 1934, Serial No. 751,192, which has matured into Patent Number 2,061,107, discloses one method of spraying or coating resistive elements.) Thereafter, the coated strip 52 is subjected to stamping or imprinting, as indicated in step II of Fig. 9.

Subsequently, in accordance with step HI, the imprinted element i2 is subjected to rolling under appreciable pressure between rollers 24 h'aving suitably uniform, polished, rolling surfaces whereby to level, lower, reduce or deform the markings of the imprinting step 11.

s The configuration resulting from the imprinting of step II is preferably in the nature of concentric ridges of ribs i8 (Figs. 1, 2, 5, 6) having sloping sides of a preferred pitch of about 60,

and otherwise formed somewhat in the nature of a machine thread, the peak or upper edge parts of which are rounded rather than sharp (see magnified view of Fig. 7), to prevent crumbling and facilitate the displacement of resistive matter under the rolling or refo step.

Such a construction and configuration I find very closely approaches ideal performance and gives results superior to anything thus far available in the nature of an element for commercial production. I

However, the imprint or surface marking may take the spiral configuration I8 as in Fig. 3, or other suitable forms. Where such other forms are contemplated, it is preferable that they result in closely contiguous or continuous ribs or ridges which will be to greater or less degree deformed or compressed under rolling pressure, and result in an approximately linear contact path or paths,

substantially radial and concentric to the axis of the contact arm, it being emphasized that the higher the overall resistance rating of they element, the more closely is it desirable to approach II-A) as was done in step I of Fig.9, and aftersuitably drying or setting by any usual means, or in oven l3, to the requisite degree of plasticity, depending in each case upon the constituency of the coating used and the physical properties of the base stripping or stock selected, the imprinted strip It." with its resistive agent I6 is rolled as shown in step IIIof Fig. 10, thus depressing, deforming, reducing, relieving, or fiattening the ribbing previously imprinted and at which the coated resistive matter will conform itself. I

The structure resulting from this alternative method gives results substantially equivalent to those effected by the method of Fig. 9. A detailed perspective of such resulting structure is shown in Figs. 8 and 8 A, wherein a base member I5 has first been stamped with ridges l8 and thereafter the resistive coating N of proper plasticity or body for rolling, is laid down and finally the coated strip and ridges i8 are rolled to give the contact surfaces 68 Fig. 8-A.

It should be particularly observed, that by either' of the forms of my method herein set forth, the resistive part of the element is pro vided with a surface having relatively recessed and standing portions in the general nature of furrows, ridges, or bossing, susceptible by virtue of shape and dimension (the latter being obviously relative to the constituency and size of cabonaceous matter in the resistive coating used, as will readily appear to those skilled in the art) to uniform rolling or similar reforming. The impressed configurations may or may not be forced into the base member under the method of Fig. 9, depending upon the degree of pressure used and the hardness of the base material. In Fig. 4, the baseis shown undeformed.

An important function of such recessed structure, in general, (as pointed out broadly in my co-pending application identified above) is that of trapping or settling-down any particles of resistive matter (e. g., carbon or graphite) which may be worn off by the movement of the contact member. In this respect, the concentric grooving of Fig. 1 herein is notably efiective, and the spiralconfiguration 88 shown in part of Fig. 3 is also desirable.

'By rolling such surface indentations, a further highly important result is achieved, namely,

I a relatively smooth, compacted contact surface contactor 32 of uniform level across the path of the contactor, and over and along the relieved portions.

The reason for this is believed to lie in the fact that the minute disturbing particles otherwise normally present on such surface are more readily displaced and dispersed by the general redistribution and distortion which takes place in the mass of resistive material (and to less degree in the base) when the surface is rolled or cornpacted, some of the displaced mass of course tending to shift toward the recessed regions, particularly from and about the peak or ridge portions of the indentations.

This latter provision of the invention also, in part, distinguishes it importantly from known art and practices.

After the processed element has been installed in a control, there is some further surfacing by the wear of the contact shoes, which in any case have relatively fine surfaces (and which are often gold plated) so that within certain limits, the performance resulting from this novel structure tends to improve with use, in contrast to the old types where the wearng down of one irregular spot merely exposed others and thus contributed little if anything toward improving continuity.

It will be further observed that with this novel element, the contact shoes, whether of the fiat, button type of Fig. 2-A or the arcuate-faced, rocking type of Figs. 4 and 7A, cannot establish the haphazard contact usual with the prior, rough-surfaced elements.

Perfect continuity is possible with the present element not only because the surfaces actually contacted are rendered relatively free fromimperfections, but also because a plurality of such surfaces lie in a path radial to the axis of the contacting means-as well as concentrically progressive thereto.

This being one of the highly important features of the invention, it cannot be too greatly emphasized that the continuity sought should lie directionally along the path of rotation of the contactor, and, when the contactor is at rest or in motion there should also be continuity in a radial direction. This is intended to be shown quite clearly in Fig. l, where a rocking type of has its face curved convexly in the direction of its travel, so thatthe actual contact area of the shoe is the narrow hand 22 indicated in dotted lines.

In this figure, contact s shown made along a radial path h with the three tracks or ridges 58, (see also the sectional view of Fig. 7A) and it will be appreciated from mere inspection of this figure how continuity along the arcuate path of travel may he maintained with either or all of the three ridges i8, and the processed surfaces lac. It should also he observed in this figure the manner in which the three radial points (surfaces) of contact along the path 6; tend to sta-- hilize the shoe in this (radial) direction.

In Fig. L'E show a novel resistance element, made in accordance with the provisions of my invention, which comprises an annularly shaped base member it of insulating material, having co-extensive connection tales it, and on said member a layer of carbonaceous resistive suicstance it, in which are deformed or repressed rihhings or channels is, arranged concentrically, but branching onto the said tabs 28, where good, low resistance, electrical connection may he made with lugs 353, (which are adapted to he crimped or otherwise secured to these tales as in by means of a relatively heavy coating of low recircumferentially or sistance' carbonaceous paint iii applied in the region of these tabs 2'8, and which overlies the high resistance coating ill in and about the grooves l8 and under the riveted portions of lugs 39. (For a more particular discussion of this construction, reference is made to my co-pending application mentioned above.)

Where a variable overall resistance gradient is desired, successive layers of resistive material ll (Fig. 5) are applied over the basic, high-resistance layer it, with'suitahle low-resistance connection being made in the manner just described, to corresponding connection lugs Bil, Sill or 38 on tabs 23,

An enlarged study of the channels'or tracks it is particularly shown in the perspective section of Fig. 2, which represents the annular punching 55, out along the line 2-2 oi Fig. l, and is intended to illustrate the raised initial condition of these markings it before being rolled, (see also Figs. 7 and '7A) showing the insulating base 55, the resistive layer Ill, and the imprinted channels or tracking 58.

In Fig. 2-A, the section of Fig. 2 is shown, after a rolling or reducing of ridging is, which results in a narrow contact path or surface te which is very nearly fiat and practically free from superficial particles and irregularities. A control contact member or shoe 32 on contact arm 34 shows relatively the nature of the multiple radial and arcuate or concentric contacts established thereby with the pressed or relieved surfaces 58 on the ridges id.

The relative disposition of elementary parts, including a form of my novel resistance element, as embodied in an operative control unit, is illustrated in Fig. 6, showing a mounting member 5, a resistant element including the base l5, resistive coating H, and channeling l8 therein, and a contact shoe 32 on arm 3t rotatable about the shaft 35. This view also shows the secondary low-resistance terminal coating l9 leading under lugs 30 As already pointed out, my invention is not restricted to the preferred concentric ridging shown in Fig. 1, but may be modified to include other desirable configurations and conformations accomplishing substantially the same objects. Such contact surfaces are desirably the result of deforming, or compressing resistive matter which has been either formed about predetermined basic conformations, or has been pressed into corresponding configuration, subsequently to be further treated in accordance with my method.

A further modification in configuration, already referred to in part, and one that results in an element comparable in performance to my preferred form of Fig. 1, is that of a slowly progressive spiral i53 generated relatively about the axis of the contact shaft or corresponding center of the element itself, as illustrated in- 'rig. 3. Somewhat the same results are achieved where the concentric circles of Fig. l are laid slightly oil center relative to the axis of the control shaft.

The spiral form of track or ridge has the additionally desirable feature of cleansing or crushing e contact shoe and preventing formation oi grooves therein, for the reason that the ridges progress not only in an arcuate sense, but also angularly across the face of the shoe in cycles as it travels about its axis, in a manner simulating a sweeping motion which tends to push adhering carbonaceous dust off the shoe face.

It should he remarked that, in general, not only should the several rolled surfaces resulting from a given conformation or configuration be relieved of minute superficial irregularities, but they should stand at substantially uniform level throughout the contact path to prevent any shifting of the contact shoe along radial lines, as from an inner ring surface to an outer one (see Figs. 2A, 4 and 7-A), since such shifting, whether due to superficial particles or dimensional variations, is a source of serious derived circuit disturbance. Hence the rolling or reforming process must also be directed toward this end.

While I have illustrated and particularly described a preferred method and preferred resulting structure in accordance with the provisions of my invention, as well as suggested modifications thereof, it will be apparent that further modifications might be made without departing from the scope of the invention, and I do not desire to be restricted or limited to the precise details set forth herein, except as may be provided in the appended claims, which I desire to protect by Letters Patent.

I claim:

1. A method of making resistance elements which comprises applying a resistive coating of carbonaceous paint to a surface, heat-curing said hard dense film, stamping ridge-like grooves therein, and rolling said stamped portions to produce substantially fiat, contiguous contact surfaces thereon.

NEWTON C. SCHELIENGER. 

